Enums, Singletons and Deserialization

Question

Enums are considered best way for singletons and one of reasons for this is that it implicitly inherits Serializable.

But how enums prevents de-serialization problem of singletons?

Enum is a singleton but what are the problems of deserialization of singletons ? — clinton, Nov 04 '12 at 15:29
@clinton: if you make a singleton serializable, and don't care about how it's deserialized, you and up with a new instance of your singleton each time it's deserialized, which breaks its singletonness. — JB Nizet, Nov 04 '12 at 15:42
@JBNizet thanks for the info. I had not looked up that issue regarding singletons before now. Or it may not have struck me as important. — clinton, Nov 04 '12 at 16:30

score 7 · Accepted Answer · answered Nov 04 '12 at 15:41

The serialization mechanism handles them in a special, specific way. But traditional singletons can be deserialized fine by defining a readResolve() method that returns the unique instance. See http://www.oodesign.com/singleton-pattern.html for an example.

score 1 · Answer 2 · answered Nov 04 '12 at 17:39

1

Serialization as an argument for using enum for singleton is nonsense.

If the enum singleton is stateful, the state is lost during serialization/deserialization.

If the singleton is stateless, who cares about its identity?

answered Nov 04 '12 at 17:39

irreputable

If the singleton is stateless, and the code uses (for example) `if (someVar == SomeSingleton.INSTANCE)`, you really care about its identity. – JB Nizet Nov 04 '12 at 18:11
There's no real world use case. Jushua Bloc made this thing up. Usually a singleton should NOT be serializable anyway, using enum to facilitate serialization is just wrong. Also, singleton is an implementation strategy, not something we should publicize at interface. – irreputable Nov 04 '12 at 18:57

2 Answers2